demux_mkv: handle TrueHD properly

Apparently, Matroska packs TrueHD packets in a way lavc doesn't expect.
This broke decoding of some files [1] completely. A short look at the
libavcodec parser shows that parsing this ourselves would probably be
too much work, so make use of the libavcodec parser API.

[1] http://www.cccp-project.net/beta/test_files/mzero_truehd_sample.mkv